When blending with four input images, it produces a single file output, which appears to be a mask for one of the four files. Shouldn't there be four files?
Yes, there should be four images, but each one generated is overwritten by the next one. I hope there is a way to type a filename for mask, like mask_%c.tif or %filename_mask.tif, so each one would be saved with different filename. i'm not familiar with console aplications, so i don't know if it should be done this way.. the way i got those masks was, that i've been renaming each mask right after it was generated, but i know there must be a better way :)
Praksant is correct. The expectation is that if you care to edit the mask, you will blend only two files at a time. I didn't think many people would be interested in doing this so I didn't add a lot of sophistication.
Having the ability to dump all the masks (not just one) from a blend would be very handy for those of us that like to touch up the end product in photoshop/gimp. This type of facility is common in PTGui